1. Field of the Invention
The present invention relates to a spreading code estimating apparatus and a spreading code estimating method in a radio communication system using a CDMA system.
2. Description of the Related Art
In the CDMA (Code Division Multiple Access), a signal of a wide band subjected to a secondary modulation using a spreading code is radio transmitted on a transmitter side. On a receiver side, a received signal is multiplied by the same spreading code as the transmitter side so as to obtain a signal of a narrow band.
In the radio communication system using the CDMA system, when a plurality of transmitting stations is present, each station transmits the signal using the spreading code peculiar to each station. For this reason, the receiver side must check the correlation between all spreading codes used in the system and the phases in connection with the received signal, and estimate the spreading code of the received signal and the phase thereof.
The following will explain the conventional spreading code estimating apparatus with reference to the block diagram shown in FIG. 1. In the explanation set forth below, the same spreading code as the spreading code by which the signal is multiplied on the transmitter side is referred to as xe2x80x9creal spreading code.xe2x80x9d Then, the spreading code estimating apparatus aims to estimate the real spreading code from all spreading codes used in this system.
As shown in FIG. 1, the conventional spreading code estimating apparatus comprises a radio receiving section 1, a code number storing section 2, a spreading code generator 3, a correlation value computing section 4, an averaging section 5, and a maximum value detecting section 6. The radio receiving section 1 receives a signal of a radio frequency, and outputs a baseband signal. The code number storing section 2 stores numbers (hereinafter referred to as xe2x80x9ccode numbersxe2x80x9d) corresponding to all spreading codes and the phases used in the system. The spreading code generator 3 outputs a replica code corresponding to each code number. The correlation value computing section 4 computes a correlation value between the input baseband signal and the replica code. The averaging section 5 averages the number of correlation values necessary for sufficiently restraining noise. The maximum value detecting section 6 estimates the replica code having a maximum average correlation value as a real spreading code.
The signal of the radio frequency received from an antenna is modulated to a baseband signal by the radio receiving section 1, and is output to the correlation value computing section 4.
At the same time, a code number is output to the spreading code generator 3 from the code number storing section 3. Then, a replica code, corresponding to the code number, is output to the correlation value computing section 4 from the spreading code generator 3.
Next, the correlation value computing section 4 computes a correlation value between the baseband output from the radio receiving section 1 and the replica code output from the spreading code generator 3, and outputs the computed correlation value to the averaging section 5.
The correlation value is averaged by the averaging section 5 using the number of correlation values necessary for sufficiently restraining noise. Then, the average correlation value is output to the maximum value detecting section 6. The maximum value detecting section 6 detects the maximum value of the average correlation values of all spreading codes used in the system, and estimates the replica code corresponding to the maximum value as a real spreading code.
In the above-explained spreading code estimating apparatus, however, the following problem exists when the number of spreading codes to be estimated and that of phases are large.
More specifically, time for estimating the spreading codes increases in averaging the correlation values of all spreading codes using the number of correlation values necessary for sufficiently restraining noise.
An object of the present invention is to provide a spreading code estimating apparatus capable of shortening spreading code estimation time even if the number of spreading codes to be estimated and that of phases are large, and to provide a spreading code estimating method.
The object of the present invention can be attained by averaging correlation values smaller than the number of the correlation values necessary for sufficiently restraining noise so as to select spreading codes as candidates, and by averaging correlation values of the selected spreading codes using the number of correlation values necessary for sufficiently restraining noise so as to specify the spreading code.